FIG. 1 depicts a schematic diagram of a portion of a typical wireless telecommunications system, which provides wireless telecommunications service to a number of wireless terminals (e.g., wireless terminals 101-1 through 101-3) that are situated within a geographic region. The heart of a typical wireless telecommunications system is Wireless Switching Center ("MWSC") 120, which may be also known as a Mobile Switching Center ("MSC") or Mobile Telephone Switching Office ("MTSO"). Typically, Wireless Switching Center 120 is connected to a plurality of base stations (e.g., base stations 103-1 through 103-5) that are dispersed throughout the geographic area serviced by the system and to the local- and long-distance telephone offices (e.g., local-office 130, local-office 138 and toll-office 140). Wireless Switching Center 120 is responsible for, among other things, establishing and maintaining calls between wireless terminals and between a wireless terminal and a wireline terminal, which wireline terminal is connected to Wireless Switching Center 120 via the local and/or long-distance networks.
The geographic area serviced by a wireless telecommunications system is divided into spatially distinct areas called "cells." As depicted in FIG. 1, each cell is schematically represented by a hexagon; in practice, however, each cell has an irregular shape that depends on the topography of the terrain surrounding the cell. Typically, each cell contains a base station, which comprises the radios and antennas that the base station uses to communicate with the wireless terminals in that cell and also comprises the transmission equipment that the base station uses to communicate with Wireless Switching Center 120.
For example, when wireless terminal 101-1 desires to communicate with wireless terminal 101-2, wireless terminal 101-1 transmits the desired information to base station 103-1, which relays the information to Wireless Switching Center 120. Upon receipt of the information, and with the knowledge that it is intended for wireless terminal 101-2, Wireless Switching Center 120 then returns the information back to base station 103-1, which relays the information, via radio, to wireless terminal 101-2.
When wireless telecommunications system 100 is a terrestrial system, in contrast to a satellite-based system, the quality and availability of service is subject to the idiosyncrasies of the terrain surrounding the system. For example, when the topography of the terrain is hilly or mountainous, or when objects such as buildings or trees are present, a signal transmitted by a base station can be absorbed or reflected such that the signal quality is not uniform throughout the cell. The result is that some areas of the cell might receive little or no signal from the base station because they are in the shadow of a mountain or a building. Alternatively, some areas can receive a direct path signal and one or more reflected signals from the base station such that the signals destructively interfere in accordance with the well-known multipath problem. In either case, for the purposes of this specification, a region of poor signal quality is called a "fade."
FIG. 2 depicts a schematic diagram of cell 102-1 of FIG. 1, which contains base station 1031, wireless terminal 101-1, fade 201-1 and fade 201-2. In general, the shape, intensity and area of a fade is based on terrestrial features.
At the perimeter or "penumbra" of a fade, the signal quality is typically diminished a little such that a call between the wireless terminal and the base station can continue, although generally with either noise or lapses in the conversation. At the interior or "umbra" of the fade, the signal quality is typically so attenuated that the base station and the wireless terminal cannot communicate at all.
When the user of a wireless terminal carries the wireless terminal into the umbra of a fade while engaged in a call, the call is invariably dropped, which annoys the parties to the call and deprives the operator of the wireless telecommunications system of revenue. Therefore, the need exists for a technique for eliminating or ameliorating the effect of a fade, or of preventing the user of a wireless terminal from carrying the wireless terminal into the umbra of a fade while engaged in a call.